Accurate alignment of a subject's eye is important in a number of situations. For example, when taking certain types of eye measurements, it is critical to know that the eye is in a particular reference position. When measuring the cornea of a patient's eye before therapeutic treatment, it can be important to repeat those measurements after the treatment to determine how much, if any, the treatment has affected the measurements. In order to accomplish this, one must ensure that the eye alignment is in the same position each time the particular measurements are made. Otherwise, the difference in data from before and after the treatment might be due to a change in eye alignment rather than the treatment.
In addition to those situations where one needs to ensure that the eye is aligned in the same position for two or more measurements, there are situations where eye alignment is desirable for diagnostic measurements of eye performance. There are situations when a human subject can simply be requested to fixate on a particular object. Thus, the human may state that he or she is currently looking at a light source, thereby providing “subjective” eye alignment information. However, there are situations where a physician or researcher would like “objective” eye alignment information indicating the orientation of the eye and, to the extent possible, indicating what the eye is viewing.
For example, very young children cannot be relied upon to fixate on such an object for measurements, such as refraction measurements which are very desirable to ensure that “in focus” images are being received when the child's brain is learning to interpret images. Likewise, adults subjected to extended eye examinations may become tired or subject to other duress and fail to maintain reliable fixation. A patient who is subjected to a therapeutic process such as laser ablation eye surgery may not be able to maintain desired eye orientation over an extended treatment time because of applied anesthesia, fatigue, or distraction by the procedure. Further, a research animal typically cannot be trained to fixate during eye measurements.
In each of the above cases, the failure or inability of the subject to maintain eye fixation upon an object can produce eye measurements or treatments that are seriously in error. Therefore, there are situations where absolute eye alignment data is needed (i.e., the eye is aligned in a certain manner) and situations where comparative eye alignment data (i.e., the eye is in the same alignment as when earlier measurements were taken) are needed and one cannot rely upon a subject maintaining the alignment.
A high level of accuracy is often required when performing surgery or other treatment on a part of the body that is subject to involuntary movement. It is typically a problem to align a patient's eye. The eye is predisposed to saccades, which are fast, involuntary movements of small magnitude. A patient may voluntarily shift their gaze during surgery, and furthermore, eye position stability is affected by the patient's heartbeat and other physiological factors. Moreover, there is still debate regarding the proper reference axis for alignment of the eye for treatment, such as laser refractive surgery.
In typical laser ophthalmic systems for treatment of defects or conditions, an eyetracker component of the system is utilized to track the motion of the eye during surgery, and to interrupt delivery of the therapeutic treatment when tracking cannot be maintained. Often, the surgeon will engage an eye tracker manually when it appears to be properly aligned. This subjective technique is prone to error which may lead to decentered ablations and other impediments to satisfactory vision correction. Various eye tracker technologies are commercially available. In some embodiments of the present invention described below, it is desirable to engage an eye tracker when it is locked onto the desired reference point on the eye.
Various types of visual axis detecting devices have been proposed. For example, some visual axis detecting devices are based on the patient's gaze. Japanese Patent Publication 1-274736, for example, describes a device which projects parallel light beams to an eyeball of an observer from a light source and determines a visual axis by making use of an image reflected from a cornea, that is, a cornea reflected image, or Purkinje image, and the imaging position of a pupil.
Thus, there is a need for more reliability and accuracy in eye alignment, particularly as it relates to eye treatment methods such as laser ophthalmic surgery.